Internal combustion engines may utilize an exhaust system that includes a selective catalytic reduction (SCR) catalyst for reducing the amount of NOx that is ultimately discharged to the surrounding environment during operation of the engine. An SCR catalyst may utilize a liquid reductant such as an aqueous urea solution that is injected into the exhaust gasses upstream of the SCR catalyst. Prior to reaching the SCR catalyst, the water droplets within the injected solution may evaporate. The remaining urea component then hydrolyzes and decomposes into ammonia which then enters the SCR catalyst via the exhaust gas flow stream. A catalyst within the SCR catalyst facilitates a reaction between the NOx component of the exhaust gas flow stream and the ammonia to break down the NOx into water vapor and nitrogen gas. The efficiency of this NOx reduction is directly proportional to the degree of vaporization of the aqueous urea solution and uniformity of the distribution of the resulting ammonia within the engine exhaust gasses upstream of the SCR catalyst.
Various systems have been used as an attempt to address incomplete vaporization and distribution of the liquid reductant within the exhaust gas flow stream prior to entering the SCR catalyst in non-stationary or vehicle related applications. The dimensional constraints of such applications have traditionally not allowed for the requisite mixing length between the injector of the liquid reductant and the SCR catalyst to achieve sufficient vaporization and uniform distribution of ammonia across the exhaust gas flow stream profile. For example, where the liquid reductant is not sufficiently vaporized by the exhaust gasses before reaching the catalyst within the SCR catalyst, drops of liquid may be deposited onto the catalyst, which may leave residue upon evaporation and eventually lead to degradation of the catalyst.
In one approach, a system for treating exhaust gasses from an engine, the exhaust gasses routed from the engine to atmosphere through an exhaust passage, is provided. The system comprises an injector directing a spray of liquid reductant into exhaust gasses against a flow of exhaust gasses; a spray target, the spray target having a front and rear surface, where the front surface is convex and generally facing a direction opposite of exhaust flow, the spray target positioned in the passage such that injected reductant impinges on the rear surface of the spray target to increase reductant vaporization; and a selective catalytic reduction catalyst positioned downstream of the injector and downstream of the spray target.
In this way, by injecting a liquid reductant on to a rear surface of a spray target with a convex front surface that generally faces a direction opposite of exhaust flow, backpressure on the engine may be reduced while also enabling sufficient vaporization and uniform distribution of the liquid reductant within the exhaust gas flow stream prior to entering the SCR catalyst.